Textures with many channels

At least 5 obvious texture channels are desirable:
- Red
- Green
- Blue
- Transparency
- Object colour

In addition, it may be desirable to use further channels:
- Mirror
- Inverted(<any channel type>)
- Additive(<any channel type>)
- Variable

Any perhaps even further complicated types of channels:
- Filter(<any channel type>, <colour filter>)
- Time-based visibility/effect

Not many image formats support more than 4 channels, but most support metadata. Current proposal is to use metadata to define meanings for each channel (with obvious defaults for 1-4). If there are more than N channels defined (where N is the maximum number of channels natively supported by the format; 4 for PNG), the image is divided in half (horizontal or vertical) and the second half mapped to channels N+1..N*2. If there are more than N*2 channels defined, the image should be divided in 3 segments, and so on. If there are more than N*3 channels, it may be desirable to divide both horizontally and vertically, using quarters.

FIXME: How does this apply to YMCK and other colourspace images?